Conventional thermoset resin compositions are generally low-molecular weight compounds which are converted into a cross-linked high-molecular weight resin by a curing reaction. Conventional thermoset resins are widely used in view of their generally advantageous properties. For example, they are easily molded into shaped objects of the resins in the uncured state and yet possess high strength and rigidity after curing. On the other hand, as the curing reactions are irreversible, the conventional thermoset resins be re-molded once they have been cured. Thus, they generally lack recyclability.
S. A. Canary et al. (J. Polym. Sci. Part A: Polym. Chem. 30 (1992) p. 1755) disclosed cross-linked resins comprising polymer chains which are connected to one another via Diels-Alder adducts obtainable from a furfural derivative such as a diene and a maleimide as a dienophile. The Diels-Alder adducts are formed from the diene and the dienophile and reverse to the diene and the dienophile when the resins are heated at an elevated temperature. Hence, upon heating at elevated temperature the cross-links disappear, allowing the resin to be re-molded into a shaped object. Practical obstacles to recyclability of the; resin still exist.
Similar cross-linked resins, based on Diels-Alder adducts of (2-furyl)-2-ethyl groups and maleimide groups attached to a polymer back-bone, have been disclosed by Y. Chujo et al. (Macromolecules 23 (1990) p. 2636).
Cross-linked resins based on furfuryl groups attached to a polymer back-bone and a bis-maleimide cross-linking agent have been disclosed in DE-A-4104319. It appears that the resins of this reference need a relatively high temperature for (re)molding. EP-A-372602 discloses a resin which is obtainable by reacting a Diels-Alder adduct of a polymeric 2,5-dialkyl furan and maleic anhydride with an epoxide. The reaction of this Diels-Alder adduct with an epoxide opens the cyclic anhydride of the Diels-Alder adduct and renders thereby the Diels-Alder adduct thermally labile, such that the latter decomposes rapidly to yield the polymeric 2,5-dialkyl furan and maleic acid esters. Hence, the resin composition disclosed in EP-A-372602 is not a re-moldable cross-linked resin as now invented. In the light of the above teaching that furfuryl groups have a relatively low stability, it would be desirable that the resins could be processed at a lower temperature.